Lean-burn operation of spark-ignited internal combustion engines is desirable because it improves fuel economy. By combining lean-burn and variable cam timing (VCT) technologies in port fuel injected engines, improvement in fuel economy of about 8 to 10% can be achieved. Moreover, available data suggest that the feedgas emissions of a lean-burn VCT engine are also improved. As used herein, the term "feedgas" means the exhaust gas leaving the engine prior to any aftertreatment. And, the term VCT refers to engine cylinder valve timing control of either intake and exhaust valves or only exhaust valves.
Additional improvement in efficiency is possible by operating a direct injection spark-ignited (DISI) engine in a very lean stratified-charge mode. The present invention addresses the problem of scheduling camshaft timing, air/fuel ratio, and electronic throttle position for lean operation of both port-injection and DISI engines in order to achieve optimum performance in terms of fuel efficiency and emissions as well as the driveability or torque response of a conventional engine .
For the purposes of this specification, it is assumed that the engine is equipped with an electronic throttle control (ETC) in which the vehicle driver merely operates a potentiometer, with the actual throttle opening being determined by the engine's electronic controller. Other sensors and actuators used with conventional electronically controlled engines may be employed with a system and method according to the present invention. Performance of the present system during lean operation may benefit from a universal exhaust gas oxygen (UEGO) sensor used instead of or in conjunction with a heated exhaust gas oxygen(HEGO) sensor.
The additional degrees of freedom available in a lean-burn VCT engine make the scheduling of camshaft timing, air/fuel ratio, and electronic throttle position difficult. The method proposed in this specification is structured to decouple driveability issues from the steady sate scheduling of the ETC, camshaft timing and air/fuel ratio. The optimal steady state schedules are obtained using the engine data of fuel consumption and HC, CO, and NO.sub.x emissions at different cam and air-fuel values with engine speed and braking torque held constant. Demanded torque at a given engine speed can be achieved by many different combinations of throttle position, camshaft timing, and air/fuel ratio. The present system and method assures that, at a given torque demand, the steady state values of camshaft timing and air/fuel ratio are optimal.
Transient operation of the ETC, the air/fuel ratio and camshaft timing must be carefully managed in order to achieve torque response resembling a conventional engine. Because the ETC (as an actuator) and fuel injectors are much faster than the cam timing actuator, the following sequence is employed for scheduling and dynamic transient compensation: (a) cam timing command is as prescribed by optimal steady state schedules; (b) the ETC command contains a component which is used to compensate for the cylinder air-charge variation due to cam timing transients; and (c) the air/fuel ratio command contains a dynamic component that matches the manifold filling dynamics to avoid large torque excursions and driveability problems. In general, when valve timing is moved from a more retarded position to a more advanced position, the ETC must be placed in a more closed position; conversely, when valve timing is moved from a more advanced position to a more retarded position, the ETC must be placed in a more open position.
One distinct feature of the proposed method is that the air/fuel scheduling into the lean region is air-driven not fuel-driven. This makes the task of simulating the driveability of a conventional engine much easier because engine output torque is much more sensitive to fuel changes at constant air than to air changes at constant fuel. For example, for a fixed flow of fuel, changing the air flow from 20:1 lean to stoichiometric changes the engine torque by about 6% to 8% as this only changes the efficiency of the engine. On the other hand, for a fixed air flow, changing the fuel flow from 20:1 lean to stoichiometric changes the torque by more than 30%.
To meet legislated tailpipe emission requirements, lean-burn engines must be equipped with a "lean NO.sub.x trap" (LNT) to reduce the exhaust concentration of the oxides of nitrogen (NO.sub.x) The LNT requires periodic purging, which is accomplished by operating the engine at either exact stoichiometry or at a rich air/fuel ratio for a period of time. Changing the amount of fuel from lean to rich operation causes an increase in torque which is not demanded by the driver, resulting in driveability problems. The present air-driven method of operation avoids this problem and allows purging of an LNT without causing torque variation.
During stratified operation of DISI engines the problem of fuel-driven air-fuel control is even more pronounced and the benefits of the present air-driven scheduling is more significant.